Automatic frequency control circuits



M. G. CROSBY AUTOMATIC FREQUENCY CONTROL CIRCUITS June 21, 1949t 2 Sheets-Sheet l Filed March 8, 1946 INVENTOR NIJ/'r 66u56] f? ATTORNEY June 21, 1949. M. G. CROSBY AUTOMATIC FREQUENCY CONTROL CIRCUITS 2 Sheets-Sheet 2 Filed March 8, 1946 Manz/M740# 017/965 l ro m0514452 raf/@www2 Patented June 21, 1949 UNITED AUTOMATIC FREQUENCY CONTROL CIRCUITS Murray G. Crosby, Upper Montclair, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 8, 1948, Serial No. 652,912

(Cl. Z50-36) 4 Claims.

My present invention relates to frequency control circuits, and more particularly to an automatic frequency control (AFC) circuit of the type generally responsive to a shift in frequency of alternating current energy from a predetermined reference frequency value.

One of the important objects of my present invention is to provide an improved and all-electronic AFC system ofthe type disclosed and claimed in my U. S. Patent 2,379,689, granted July 3, 1945. The prior systems in which the detected output of a discriminator is applied directly to the reactance tube, require the continuous presence of the controlled oscillator and the reference oscillator to maintain the proper intune condition (the reference oscillator is the stable oscillator in the transmitting case, and the incoming signal in the receiving case) Absence of either oscillator in the control circuits allows the oscillator to shift to the off-frequency condition it would have taken in the absence of the control system.

As far as I know, this case is the first all-electronic type of automatic frequency control system which applies a correction voltage to the reactance tube that does not allow the oscillator to drop back to its drifted position in the absence of the control potential. This type of operation is a necessity in radio telegraph reception, since it leaves the receiver in proper tune when the transmitter goes olf the air.

Another important object of this invention is to provide a frequency control system, adapted for example for use as a servo-mechanism, wherein the system utilizes solely electronic units and is not limited by in-ertia effects.

Another important object of my present invention is to provide an AFC system which is more flexible of adjustment than the system shown in my aforesaid patent, and which is readily applicable to frequency-shift keying.

Yet another object of my invention is to provide an AFC system wherein a condenser is charged an amount which, when applied to a reactance tube, corrects detuning.

A more specific object of the present invention is to provide a frequency control system of the all-electronic type wherein there is utilized a charging capacity to provide AFC bias for an electronic reactance tube, the charging condenser being under the control of a pair of rectifiers which are normally biased to cut-off to prevent change of voltage across the charging capacity, and the voltage across the charging capacity being a function of space current flow in said rectiiiers in response to a frequency shift of control energy from a predetermined reference frequency.

Still other features and objects of my invention will best be understood by reference to the following description. taken in connection with the drawing in which I have indicated diagrammatically several circuits whereby my invention may be carried into effect.

In the drawings:

Fig. 1 illustrates a system for receiving radio signals embodying the invention;

Fig. 2 illustrates modifications required to be made in the system of Fig. 1 in order to have it applicable to a transmitter of frequency modulated (FM) signals;

Fig. 3 graphically explains the functioning of the diodes 5I and 52;

Fig. 5 shows a modification of the invention;

Fig. 5 explains the operation of the modification in Fig. 4;

Fig. 6 shows still another modification; and

Fig. 7 explains the operation of the modification of Fig. 6.

Referring now to Fig. 1 of the accompanying drawing, in which certain of the networks are schematically represented, I have shown my present AFC system applied to a receiver of a well known form. Generically, there is shown an oscillator I having a resonant tank circuit 2. The latter is provided with a tuning, or frequency selecting device 3. The oscillator, which may be considered as operative in any suitable and desirable frequency range, feeds its oscillations to a network designated as a converter. The latter network 4 is to be understood as schematically indicating the converter of a superheterodyne receiving system. As is well known to those well skilled in the art of radio communication, the converter 4 will have an input electrode connected to a suitable source of signals. The converter output energy, in the form of signal modulated oscillations whose mean frequency is the difference between the signal frequency and that of the frequency of tank circuit 2, is applied to a selective amplifier 5 having input and output circuits each tuned to the aforesaid mean frequency. The numeral 6 designates the tuned output circuit of selective amplifier 5.

The numeral 'I denotes a transmission path feeding a utilizing circuit with the output energy of the selective amplifier 5. As the system is assumed to be a superheterodyne receiver, the oscillator l will be the usual local oscillator which is tunable by the tuning condenser 3 over a range cf desired oscillation frequencies. The network il will, therefore, be the conventional rst detector or converter whose tunable input circuit is adjustable over a range of carrier frequencies. The converter output circuit will be fixedly resonated to the intermediate frequency (I. F.) value of the superheterodyne receiver, while amplifier 5 will in that case be tuned to the operating I. F. value as will, also, be the resonant output circuit 6. The path 'l may then feed the I. F. energy to addtional I. F. amplifiers, and the latter will usually be followed by a demodulator, audio frequency amplifiers and a final reproducer.

In Fig. 2 I have shown the modifications required for the invention to be applied to a transmitter, and more particularly an FM transmitter. In such case the master oscillator I' is provided with tank circuit 2 and tuning capacitance v3. The oscillator output may betransmitted over lead 2 to an amplifier, frequencymultiplier and signal radiator (all not shown since they are Well known). The converter 4' would now be fed with oscillations from master oscillator I', and, instead of signals as in Fig. 1, energy from a stable reference oscillator I" which may be a crystalcontrolled oscillator. Selective amplifier 5 would then have the LF. energy output ofy converter 4 applied thereto, and there would be no need for the output connection 'I for amplifier 5'. It is to be lclearly understood that the remaining circuit elements now to be described in Fig. 1 are equally applicable to the FM transmitter of Fig. 2.

In either system, whether receiver or transmitter, there is developed at circuit 6 modulated carrier frequency energy. The carrier frequency may be in the kilocycle (kc.) range or in the megacycle (mc.) range, and the character of the modulation may be either amplitude or frequency. As is well known in present day superheterodyne receivers, a change in oscillator frequency is concurrently accompanied by a change of the frequency of the converter selective output circuit. The resulting I. F. signal output energy is not accurately tuned in unless its mean frequency is equal to the predetermined resonant frequency of circuit 6.

The AFC circuit responds to a frequency diierence between the I. F. signal energy and the predetermined I. F. value to vary the oscillator tuning in a sense, and to an extent, such as to cause the frequency difference to be reduced. Similarly, in the case of a transmitter, a change in the constants of master oscillator I may cause a frequency difference to exist between the predetermined frequency value of amplifier 5 and the mean frequency of the modulated carrier energy output of the. amplier.

A well known form of AFC detector, or discriminator-rectiiier, is shown coupled to circuit 6.

The function of the AFC detector is to provide a control voltage for AFC purposes in response to a predetermined frequency shift of the carrier of the modulated wave energy applied to circuit 8. The specific form of circuit shown in the AFC detector is that disclosed and claimed by S. W. Seeley in his U. S. Patent 2,121,103, granted June 21, 1938. Accordingly, the action of the AFC detecto-r will not be described in detail, reference being made to that partent. It is to be clearly understood, however, that any other suitable and well known form of AFC detector may be utilized to provide the control voltage which is representative of the frequency shift of the signal energy at output circuit 6.

It will be sufficient for the purposes of this application to explain that a resonant circuit 8, tuned to the same frequency as circuit 6, is magnetically coupled thereto. A direct current blocking condenser 9 is connected directly from the high alternating potential side of circuit 6 to the mid-point of the inductance 8 of secondary circuit 8. The diode rectiers I0 and Il, which are specically shown as provided by the electrodes of a duo-diode type tube, are arranged in polarity opposition. The anodes of the diodes are con- 4 nected to respective opposite sides of the common input circuit 8. The cathodes are connected by a center-tapped resistor which is thereby divided into series-connected load resistors I2 and I2. Each of load resistors I2 and I2 is bypassed for carrier frequency currents by a respective shunt condenser.

The cathode end of resistor I2 is grounded, and the junction of load resistors I2 and I 2' is connected back to the mid-point of coil 8 through a path consisting of high frequency choke coil I3. The numeral I3 designates a source of low frequency current. For example, the latter may be the usual 60 cycle line current employed to energize the various electron discharge tubes of the system. The invention is not limited to the use off 60 cycle current, and it is to be vclearly understoodthat any other value of low frequency may be used in place of the specific 60 cycle value.

When the mean frequency of the applied modulated carrier energy is equal to the predetermined frequency of each of circuits G and 3, the radio frequency voltages applied to the rectiers ID and H will be of equal magnitude. This relationship exists by virtue of the construction of thediscrimina-tor input network, and has been explained in detail in the aforesaid Seeley patent. The equal radio frequency voltages applied to rectiers I8 and II are rectified, and equal direct current voltages are developed across resistors I2 and I2.

On the other hand, if the mean frequency of the applied modulated carrier energy departs from the predetermined reference frequency in one direction, one of the rectifiers will receive more radio frequency voltage than the other. Hence, the direct current voltage developed across one of the load resistors is greater than that across the other resistor. The differential output across the entire output resistor path I2, I2 will then no longer cancel so that the voltage at the cathode end of resistor I?. will have a magnitude and polarity which will respectively depend upon the extent and direction of the shift of the signal frequency at circuit 6 with respect to the predetermined frequency of circuits 6 and 8.

In my aforesaid patent there was utilized a two-phase motor of variable speed and variable direction for controlling the frequency of an oscillator tank circuit. In my present application the system is all-electronic, and is not limited by the inertial effects. In my all-electronic type of AFC there is applied to the reactance tube a correction voltage that does not allow the oscillator I to. drop back to its drifted position in the absence of the control potential. Where the receiver is employed in radio telegraph reception the present invention is virtually a necessity, as it leaves the receiver in proper tune when the transmitter goes off the air. Additionally, my system is more iiexible of adjustment. These various advantages are secured by means of the circuits now to be described in detail.

The AFC detector is followed by an electron discharge device which functions as aphase reverser, and is of the pentagrid tube type. The cathode I6 of the tube I5 is connected to ground through the cathode bias resistor I'I, while the first grid I8 is coupled by condenser I9 and resistor 20 to the grounded end of load resistor I2. It will be observed that the ungrounded terminal of the 60 cycle current source III is connected by lead 2I to the ungrounded end of resistor 2Q. Hence, the 60 cycle current source I4 is shunted across resistor 20, and Athe potentiometer slider 2,2 is provided for adjusting themagnitude of the 60 cycle voltage which may be applied over lead 23 to the third grid 24 of tube I5.

The grids I8 and 24 both have 60 cycle voltage applied thereto. The grid I8 has the entire alternating current voltage across resistor 20 applied thereto through condenser I9, while the grid 215 has a predetermined fraction of the voltage across resistor 29 applied thereto. In addition, control voltage, which is direct current voltage, is applied to grid I8 by means of the path consisting of resistor 25, lead 26 and resistor 21. In other words, the grid I8 returns to the grounded end of cathode resistor l1 through a path consisting of resistor Z'Llead 26, resistor 25 and load resistors I2 and I2'. The lower end of resistor 2 is bypassed to ground by condenser 35 for alternating currents.

The third grid 24 is surrounded by a positive shielding field by virtue of the fact that the second and fourth grids of tube I5 are connected in common to a source of positive voltage through the resistor 3 I. The fifth grid of the tube is preferably connected to the cathode I6, and the plate 32 is connected through the output resistor 33 to the +B terminal of the positive source of voltage.

If the voltage at the cathode end of resistor i2 is zero, it corresponds to the condition that the mean frequency of the modulated high frequency energy at circuit 6 is equal to the predetermined reference frequency of circuits 6 and d. Hence, the direct current voltage of grid I8 is determined solely by the voltage drop across cathode resistor I'I. The slider 22 is adjusted to a point on the potentiometer resistor 2i) such that when zero control voltage is applied to grid i8, the 60 cycle voltages on grids I8 and 24 have equal eiect on the space current owing to the plate S2. Hence, the 60 cycle voltage developed across resistor 33 is zero in magnitude for that condition of the control voltage developed by the AFC detector. The 60 cycle voltage developed across resistor 33 will change correspondingly in phase and amplitude in response to the variations of magnitude and polarity of the voltage at the cathode end of resistor I2. Hence, I have designated tube i5 as the Phase reverse tube to indicate that it functions to provide a 60 cycle voltage at its output terminals whose phase and amplitude are a function of frequency shifts of the modulated high frequency energy at circuit 6.

Pentagrid amplifier l5 is a negative transconductance type of tube with the screen 24 unbypassed, and acts as two amplifiers in cascade with reference to the first control grid I8. The first grid i3 and the screen 24 act as a triode. Screen resistor 3l acts as the plate resistor in this rst triode circuit. For the elements comprising the screen 24 and plate 32, the screen constitutes the control grid of a second triode which ampliiies the voltage appearing across resistor 3l. Thus, in effect, the insertion of a resistor in the screen circuit forms a combination of two triodes in cascade, so that the voltage fed to the first grid goes through one phase reversal for each of the triodes. Hence, a complete 360 rotation is obtained by the rst grid I8 and the output plate r`Ehe second grid 2li operates in conjunction with plate S2 as a simple pentode, with the single phase reversal of such an amplifier. Hence, when voltage with the same phase is fed to grids I8 and 2t, the amplied voltages of the two grids oppose in the plate circuit 32, 33. Consequently, by properly adjusting the relative vmagnitudes of the in the output circuit 33.

voltages fed to the two grids by means of potentiometer 2Q, complete cancellation is obtained When a control potential is applied to grid I8, through lead 25 and resistor 2l, the relative ampliiications of the two grids are upset so that complete cancellation in the plate circuit no longer takes place. The direction and magnitude of the control voltage determines the polarity and magnitude of the voltage due to the unbalance which appears in resistor 33. Hence, a positive control voltage produces one phase of output voltage, and a negative control voltage the opposite phase. The result is a phase reversal.

The tube llt, shown as of the pentode type, is an amplifier, and functions to amplify the 60 cycle voltage developed across resistor 33. The cathode Il! of tube li@ is connected to ground through the biasing resistor d2, and the input grid i3 is connected by the coupling condenser d to the plate end of load resistor 33. It is noted that the resistor e5 returns the grid 23 to the grounded end of biasing resistor 42. The plate l5 of amplier tube '55 is connected through the primary winding fil of transformer T to a source of positive voltage +B. rI'he screen grid of tube i5 may be connected to the same voltage source through the resistor fit, being bypassed to ground for alternating current by condenser 49.

The amplified 60` cycle Voltage of suitable phase, developed across the secondary winding 5t of transformer T, is applied to a pair of diode rectiiiers 5I and 52. While the rectiers 5I and 52 are shown specifically as diodes, it is to be clearly understood that they may be tubes of any other type. The essential thing is to utilize electron discharge devices at 5I and 52 which are adapted for connection as rectifiers of 6i) cycle voltage. The cathode 53 of diode 5I and the anode 54 of diode 52 are connected in common to the AFC lead 55, the charging condenser 56 being connected from the lead to ground. The anode 5l is connected to the negative terminal of a direct current biasing source 58, and the positive terminal of the current source 58 is connected to one end of the secondary winding 5i). The cathode 59 of diode 52 is connected to the positive terminal of the direct current biasing source 5B, whose negative terminal is connected to the opposite end of winding 50. The mid-point of secondary winding 50 is connected by lead t! and the secondary winding 62 of transformer T' to ground. The primary winding 63 of transformer T is itself connected in shunt across the cycle current source I4. Accordingly, 60 cycle voltage is applied to the mid-point 5D of winding 5B by means of transformer T.

The voltage sources 58 and 60 apply biasing voltage to each of the diodes 5I and 52 such that normally the rectiers are cut off thereby to prevent charging of the condenser 55. That is, with no amplified 60 cycle voltage applied to transformer 'I' the rectiiiers 5I and 52 are cut off, since the 60 cycle voltage applied through the transformer T' is of insucient magnitude to overcome the cut-off bias applied to each of the rectifiers. This situation exists, of course, when the voltage derived from the AFC detector and applied to grid I8 is of zero magnitude. It is in this condition that there is developed zero 60 cycle voltage across output resistor 33 of tube I5. However, when there is developed a differential voltage across the output load of the AFC detector there will be developed 60 vcycle voltage across the secondary winding 50 of transamazed 7. former T. When the voltage through trans-- former T combines with the voltage throughy transformer T' to overcome the cut-off bias. of each of the rectiers, then the condenser SA will be charged by the rectified 60 cycle voltage.

Condenser 5S is chosen to have low losses: so that it will hold its charge for a considerable length of time. If oscillator l takeson a detuned' condition, the control voltage generated across` condenser 56 is a direct current Value just suficient to shift the oscillator, by means of the reactance tube, the required amount to bring it back in tune. A condition of equilibrium is then obtained in which condenser 56 holds this xed amount of charge. Of course, a loss-less condenser is unobtainable, but, in the practicalcase, a condenser may be obtained which will hold a charge as long as required. Hence, the AFC voltage over lead 55 is not necessarily alternating voltage.

Fig. 3 shows the relation between the alternating current voltages fed to diodes 5| and 52 and the control potential. In the balanced condition the voltage fed to each diode comprises only that obtained from transformer T', which gives avolt'- age corresponding to amplitude a in Figure 3; When a control voltage is applied to lead? 26', the voltage amplified by the phase reverser and amplier opposes the voltage from transformer T' at one diode input, and aids that fed* to the other diode. As a result, the applicationof a control potential raises the input to one diode and lowers that to the other. By means of this amplifying arrangement, the resp-onse tothe control potential may be made very sensitive.

Diodes 5| and 52 are normally biased by means of batteries 58 and 50 so that current is not drawn for amplitudes of input lower than the value a. Hence, for the in-tune condition, at which both inputs will have the amplitude a', condenser 56 will remain in its uncharged; condition. When a positive control potential is ap'- plied the input to diode 5I increases, and1 that to diode 52 lowers. This causes diode 5|- to draw current, and charge condenser 56 iny a direction which places a positive potential on lead 55; When a negative control potential is applied, the input to diode 52 rises and that to 5II lowers. This causes diode 52 to draw current in a direction which tends to charge condenser 56 negatively. If a positive charge is already onI 56, the

result of this negative charge will be a reduction of thepositive charge. It is thus seen that as the control potentialis made positive or negative, the charging voltage applied to the condenser is, also, made positive or negative. For the intune condition charging current does not flow' to the condenser, and it holds to thel value'- of charge which was last applied to it.

The charging voltage developed across condenser 56 is employed to control the gain of any suitable form of electronic reactance tube; The numeral 15 denotes such a tube. The circuit connectionsbetween tube 'lil and the tank circuit 2f of the controlled oscillator I are welll known.

It is to be clearly understood that the AFC voltageI may be applied over lead 55 to any suitablev andl well known form of reactance tube circuit; With'- out going into specic details of the reactance tube circuit, it-is pointed out that the pentode 8 isconnecteditoground through a suitable biasing resistor 14.

The. tank circuit 2- is, furthermore, shunted by ay suitable phase shifter network. In this case, the phase shiftery network consists of resistor 'l5 in series with a condenser 16. The oscillatory current' ilowing im tank circuit 2V develops a voltage. across the condenser 76, which is in phase quadrature.v with thevoltage across tank circuit 2. Suitable design of the magnitudes of resistor I5 and condenserl 'I6 permits this phase quadrature shift tol be secured. It is specically desired that the magnitude of resistor 15' be relatively large compared to the reactance of condenser 76. The phase-shiitedA voltage across condenser '.'6 is applied to the grid 8U- of tube 10, and the grid is connected to the ungrounded terminal of condenser T6 through a direct current blocking condenser. The AFC lead is connected to the-grid Bilithrough the filterv resistor 9i). Suitablev positive potential is applied to plate 'H through a radio frequency choke coil 9|.

When the* tubey 1E] has normal gain, that is with no bias applied over lead 55, the plate to cathode impedance-of tube- I-f simulates an inductive reactance across tank circuit 2. The magnitude of this simulated inductive reactance depends on the magnitude of the voltage existing at the ungrounded terminal of condenserV 56. The sense of change of the inductance reactance si dependent on the polarity of the'charging voltage across condenser 5G'. Hence, there is provided an eective and simple means for change of frequency at tank circuit 2 to compensate for the frequency shift of the modulated carrier energy at circuit 6 from thepredetermined reference frequency. It is 'to be clearly understood that the control voltage applied over lead' 55 will vary the magnitude of the simulated inductive reactance in a direction and' to' an extent such that the frequency of circuit Z' is changed sufficiently to cause the frequency of" the converter output substantially to coincide withv the predetermined reference frequen-cy of circuits G and 8. It. will now be appreciated that frequency shifts at the input of the AFC detector are translated into correspondingyariations in phase and amplitudeof cycle voltage across4 resistor 33. The latter are translated. into. variations in the AFC'voltage delivered over lead' 55.

This invention is not limited. in inertia eiects, as in priormotor control. In the motortype of control a limitation in speed of operation is obtained in that as the control calls for direct changesv in direction of rotation of the motor shaft', the inertia of the motor rotor must be overcome. This inertia inherently slows. down the speed with which the control will respond to a control potential. It, also, may cause overshooting of the control which leads to hunting. In thispresent' electronic system,.however, charging. current is fedto condenser 56 in a low resistance circuit which builds up the charge on the con-denser rapidly; When the condenser i's not being' charged the diodes do not conduct, so that the condenser holds its charge for a length of time determined by its losses. The result is quick.

response to a control potential.

In thecase of the transmitter system of. Fig. 2 the lead 551s preferably' modied as is indicated. Modulationk potentials are applied to the reactance tube 'lil to eiect frequency modulation. of the carrier energy' of' the master oscillator I. This is done'byinjectingjthe modulation voltage `acruss a resistorY 55' in series in line 55. The

secondary of the input transformer M is connected across resistor 55. Those skilled in the art of radio communication are fully aware of the manner in which the modulating potentials from any desired source of modulation voltage (say audio) cause the tube l to vary the frequency of circuit 2 to create a corresponding frequency modulation of the master oscillator. The AFC circuit, being responsive to relatively slow frequency shifts at circuit t, 8 will not interfere with the modulating function.

In Fig. 4 I have shown another method of applying the principle of my invention. In addition, a different diode system is employed in place of the diode network l, 52 of Fig. 1. In Fig. 4 the selective amplifier 5 has its discriminator output circuit 6, 9, 8 coupled to a pair of electron discharge tubes loll, lill. The cathodes of tubes m0, lill are tied in common to ground, while a suitable direct current source HB2 has its ungrounded negative terminal connected to the midpoint of input coil 3 through choke coil I3. The opposite ends of coil 8' are connected to respective input grids i60 and mi of tubes Hill and lul. The respective plates lll and H2 of tubes |00 and l @l are connected to the respective primary circuits H3 and l lil of transformers |03 and IM.

A pair of double diode tubes lill and Hi8 are utilized. The tube lill has a pair of reversely connected diodes D1 and D2, while tube Hi8 is provided wth reversely connected diodes D3 and D4. The anode of diode D1 and cathode of diode D2 are connected in common through condenser |05 to the high potential side of secondary circuit I I5 of transformer idd. The cathode of diode D1 is connected to the grounded side of input circuit l i5 through the direct current source HB9. The anode of diode D2 is connected to the AFC lead 55.

The cathode of diode Ds and anode of diode D4, in the case of the second tube l, are connected in common through condenser iti@ to the high potential side of secondary circuit l I6 of transformer Mld. The low potential sides of both input circuits H5 and ii'a are grounded, and the anode of diode D3 is connected to the ungrounded negative terminal of the direct current source H3. The condenser 56 is connected from thecommon connection I l l between the anode of diode Dz and cathode of diode D4 to ground.

The diode arrangement is a counter circuit in which each half cycle of alternating current voltage applied to the input causes a small amount of charge to be applied to condenser 5G. The operation is the same as a voltage-doubler rectifier, except that condenser 05 is made small compared to 56. For instance, in the case of diode itil, the positive half cycles draw current through diode D1 which charges condenser H35. No current is drawn by diode D2 for the positive half cycle. The negative half cycle draws current through diode D2 to place a charge on condenser 56 which is dependent on the relative magnitudes of condensers H35 and 56. Since condenser M5 is usually much smailer than condenser 56, many half cycles of alternation are required before condenser 55 receives its full charge corresponding to twice the peak voltage of the applied alternating wave. Hence, the applied voltage must be left on a certain length of time to bring condenser 5% up to its full charge.

Amplifiers im! and itil take the outputs of iscriminator 6, 8, and amplify them in accordance With the characteristic of Figure 5.

Bias is applied from battery |02 so that amplilier Hit produces output only for frequencies above the carrier frequency Fc. For lower frequencies, the discriminator output does not overcome the bias. Amplifier lill amplifies the other half of the discriminator output, so that it furnishes input to diode IBB that is Zero for frequencies above Fc and rises as the frequency is made lower than Fc. As a result of this cutoff adjustment of the ampliers, a frequency shift away from frequency Fc feeds input to either diode itil or 08 depending upon the direction of frequency shift. This charges condenser 56 with a voltage which depends on the direction and magnitude of the frequency shift. For the in-tune condition neither diode draws current so that the condenser remains at the charge it iast received.

The function of batteries |69' and il@ is to apply a delay bias to diodes D1 and D2 so that condenser 5d does not discharge to ground through the series circuits comprising diodes Dz, D1 and D4, D3. This delay bias has a magnitude approximately equal to the magnitude of voltage charge applied to condenser 56. For instance, if the maximum range of positive charge applied to condenser 55 is 5 peak volts, battery itil should have a value of 5 volts. Likewise, if the maximum negative excursion of condenser 56 is 5 volts, battery Hi) should have a value of volts. (This same relation exists in the circuit of Figure 1 in the case ofbatteries 5S and eil). The result of this delay bias is to produce an idle interval in the control potentials applied to condenser 56. However, the idle interval may be made negligibly small by the application of high levels of diode input from amplifiers lim and lill.

The present system of AFC, as shown in Fig. l, is applicable to frequency-shift telegraphy reception as disclosed in my application Serial No. 491,106 filed June 17, 1943, now Patent Number 2,462,470, dated February 22, 1949. In that patent I have generally disclosed an improved teleraph receiver for FM telegraph signals which includes an AFC circuit which maintains the tuning of the receiver at a fixed frequency with respect to either the mark or the space frequencies, and which frequency control circuit will not tend to follow the mark or space frequency. There is employed means for converting the output of a conventional FM detector, or discriminator-rectier, from the square wave form that is obtained in frequency shift telegraphy reception, to a control potential which is proportional to the deviation in mean frequency of the received signal. This is accomplished by means of a limiter and combiner circuit, as schematically represented in present Fig. 6.

The rectangle 2E!! is representative of a conventional FM detector preceded by a suitable I. F. input circuit, say of the type shown in Fig. 5 of last-mentioned application. It delivers normally a square wave output for telegraph reception, as disclosed in said application. Such a square wave output is not suitable as an AFC potential. Hence, by limiting the output of detector 2Q! in a suitable limiter 202, and recombining with the output of the detector in phase opposition in combiner 203, the characteristic at the output terminals appears as shown in Fig. 7. It will be noted that limiter 2M and combiner 263 are schematically represented. Reference is made to my last-mentioned application for specific circuits which may be employed,

and more especially reference is made to Fig. of said application. The output voltage at leads 309 may be utilized for connection to resistors 25 and 20 respectively of present Fig. 1. That is, the output from leads v3l'lll is equivalent to the output across I2, I2 in Fig. 1. The benets of the present AFC method are thus applicable to the telegraph receiver of my aforesaid application.

In further explanation of this application of my invention, it W-ill be noted that this discriminator characteristic has two values, F1 and F2, at which the in-tune condition is obtained. The telegraph signals would normally be tuned so that the space frequency isat the F1 position, and the mark frequency -at the F2 position, or vice Versa. With this type of discriminator characteristic, a drift in the frequency produces a control potential which is proportional to the drift, regardless of whether the keying is in the mark or space condition. The result is a control potential for automatic frequency control which has the `saine characteristic as that obtained directly from the discriminator when the ordinary program, .or voice modulation is used.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination with ahigh frequency oscillator having means for adjusting the frequency thereof, an electronic reactance tube including connections to the oscillator for providing control over the oscillator frequency, a source of alternating voltage, a pair of rectiers connected in series-aiding relation, means for individually biasing the rectifiers Ato cut-off, -means for applying alternating voltage from -said source to both of said rectiers, a discriminator rectifier network having an linput circuit responsive to oscillator frequency shifts, said network including an output circuit constructed and arranged to develop a direct voltage representative of said frequency shifts, an electron control device having at least two spaced control electrodes, means for applying alternating voltage from said source to said spaced electrodes in such relative magnitude that output voltage derived therefrom is normally zero, means for applying said direct voltage to at least one of the spaced control electrodes, means for applying to said rectiers output voltage derived from the control veffect of said spaced control electrodes, and additional means responsive to the joint output of said rectifiers for controlling the effect of said reactance tube.

2. In a system of the type including an oscillator provided with a tank circuit whose frequency is to be controlled, a converter adapted to have signals and oscillations from said oscillator applied thereto, and a'selectiveam'plifier responsive to the converter output, a detector for translating frequency shifts of the output ofthe selective amplifer into corresponding control voltage variations, a source of lonr .frequency voltage, means responsive to said low l'frequency voltage and said control voltage variations for providing effective low frequency voltage whose phase and amplitude is representative of said frequency shifts, a pair of rectifier devices, means for normally rendering 12 said rectifier devices inoperative, means for applying said low frequency voltage from said source as a reference voltage to said rectifier devices, means for applying the variable phase and amplitude low frequency voltage to said rectifier devices thereby to provide a resulting voltage whose magnitude and polarity is a function of said frequency shifts, and means for using said resulting voltage to control the frequency of said tank circuit.

3. In a system' of the type including an oscillator provided with a tank circuit whose frequency is to be controlled, a converter adapted to have signals from a signal source and oscillations from said oscillator applied thereto, and a selective amplier responsive to the converter output; a detector for translating frequency shifts of the output of the selective amplifier into corresponding control voltage variations, a source of low frequency voltage, means responsive to said low frequency voltage and said control voltage variations for providing effective low frequency voltage whose phase and amplitude is representative of said frequency shifts, a pair of rectifier devices, means for normally rendering said rectifier devices inoperative, means for applying said low frequency voltage .from said source as a reference voltage to said rectifier devices, means for applying the variable phase and amplitude low frequency voltage to said rectifier devices thereby to provide a resulting voltage whose magnitude and polarity is a function of said frequency shifts, means for using said resulting voltage to control the frequency of said tank circuit, said means responsive to the control voltage and said low frequency voltage comprising an electron control device having at least two control electrodes, means for applying the low frequency voltage to said control electrodes in different magnitudes, and an additional connection for applying said control voltage to one of said control electrodes.

4. In combination with a high frequency oscillator having means for adjusting the frequency thereof, an electronic reactance tube including connections to the oscillator for providing control over the oscillator frequency, a pair of rectifiers connected in series-aiding relation, means for individually biasing the rectifiers to cut-off, means for applying alternating current voltage to both of said rectiers, a discriminator rectifier network having an input circuit responsive to oscillator frequency shifts, said discriminator network having an output circuit in which is developed a rectified voltage representative of said frequency shifts, means for applying said developed rectified voltage to said first-named applying means to vary the alternating current voltage applied to said .rectifers as a function of said frequency shifts, capacitive means connected to be charged by the output of said rectiers, and means connecting said capacitive means to said reactance tube to control the effect thereof in response to the charge on said capacitive means.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the file of this patentz' UNITED STATES PATENTS Number Name Date 2,250,284 Wendt July 22, 1941 2,312,079 Crosby Feb. 23, 1943 

